The invention generally relates to retainers used in stamping operations and, in particular, to retainers that rely on a ball-lock mechanism for retaining a tool during stamping operations and methods of retaining a tool in a ball lock retainer during stamping operations.
Conventional punch and die assemblies include an interchangeable punch retainer that secures a punch with a punch holder of a die set and an interchangeable die retainer that secures a die button with a die holder of the die set. One common type of retainer used with either punches or die buttons is a ball-lock retainer. The ball-lock retainer includes a metal block perforated by a primary passageway for receiving either the die button or the punch and a secondary passageway that intersects the primary passageway at an inclined angle. A spring-loaded locking ball, which is disposed in the secondary passageway and is spring biased by a compression spring within the secondary passage, engages a ball seat on the punch or die button. The engagement between the locking ball and the ball seat securely holds and aligns the punch and die button during use.
Ball-lock punch and die button retainers possess various attributes that contribute to their ongoing popularity. The punch and die button can be easily and rapidly removed from the respective retainer and replaced. Broken punches can be conveniently replaced while the punch retainer remains held in the press. However, ball-lock punch and die button retainers also have problems that limit their usage.
One particular problem observed in conventional retainers relates to ball bounce and vibration, which is experienced primarily when stamping thick materials or hard materials such as high strength steels. The punch undergoes cyclic compression and tension during the piercing and stripping operation of the press cycle. Despite the presence of the biasing force applied by the compression spring to the locking ball, the cyclic compression and tension cause the punch to move axially along its length, and the shock generated at impact and snap-through during the piercing operation may cause the locking ball to bounce and vibration. As a consequence, burrs may form on the locking ball and even the ball seat. If the extent of the burring is extreme, the punch may become difficult or impossible to separate from the retainer. Another result of ball bounce and vibration is an enhanced potential for punch pull-out from the retainer if the force of the moving locking ball overcomes the spring pressure that assures that the locking ball will remain seated in the ball seat.
Additional consequences of the ball bounce and vibration may be observed. For example, the shape of a perforation made by the working end of the punch may be altered by radial and/or lateral movement of the shank of the punch. In addition, ball bounce and vibration increase the likelihood that the locking ball will fatigue and break. If the locking ball is broken, the punch may fail or may be pulled out of the retainer during the stripping operation.
A booster spring may be added to the ball-lock mechanism that supplements the resilient biasing applied to the spring-loaded locking ball. However, such booster springs fail to provide an adequate solution to the multiple problems resulting from ball bounce and vibration during stamping operations.
Therefore, improved retainers are needed that control the motion of the locking ball of a ball-lock retainer relative to the ball seat during stamping operations.